With new legislative requirements in Europe and proposed requirements in other parts of the world, the importance of tire weight and improved fuel efficiency is more important than ever. A number of technical approaches have been taken to address these problems. One area of technology under investigation is to replace the current steel beads with aramid beads. This technology has the potential to reduce the weight of the tire. The present exemplary embodiment relates to development of aramid beads for tires.
A tire generally comprises: a carcass structure; a tread band in a position radially external to the carcass structure; a belt structure interposed between the carcass structure and the tread band. A tire generally further comprises a pair of sidewalls applied to the carcass structure in axially opposite positions. The ends of the at least one carcass ply are folded back or secured to two annular reinforcing elements, i.e. the so-called “bead bundles”, and the tire region which comprises the bead bundle is known as “tire bead”. Typically, in a position radially external to the bead bundle, the tire bead further comprises an elastomeric insert, conventionally called “bead filling” or “bead apex”, which extends radially outwardly from the respective bead bundle.
A tire bead performs the function of anchoring the tire to a respective wheel rim thereby ensuring, in case of a tubeless tire, a sealing effect between the tire and the wheel rim, the latter being provided in correspondence of the bead mounting position and generally comprising two substantially conical coaxial surfaces which act as the supporting base for the tire beads. The surfaces generally terminate in a flange, radially projecting outwardly, that supports the axially outer surface of the bead and against which the latter abuts by virtue of the tire inflation pressure.
The tire bead is required to withstand relevant deformations that arise during the fitting operation of the tire on a respective wheel rim. In fact, the diameter of the radially internal annular surface of the bead bundle is smaller than the radially external diameter of the rim flange and is chosen so that, once the tire bead has been positioned in the respective bead seat of the rim, after passing over the flange, it is pushed by the pressure of the tire inflating fluid along the diverging surface of the bead seat against the axially internal surface of the flange. Generally, the fitting of a tire on a respective rim starts with the deformation (ovalisation) of the tire bead so that a portion thereof is able to pass over the flange. Successively, the rest of the tire bead is caused to completely pass over the flange such that the bead is positioned in the closest bead seat. Then the bead is pushed axially towards the opposite bead seat so as to cause it to fall into the central groove of the rim. In this way, once the bead is located inside the abovementioned central groove, the equatorial plane of the tire may be inclined with respect to the equatorial plane of the rim so as to allow also the opposite bead to pass over the flange and be positioned in the corresponding bead seat, by means of ovalisation thereof (and hence of ovalisation of the respective bead bundle).
Finally, the tire is inflated so that both the beads come into abutment against the axially internal surfaces of the flange. Owing to the rigidity of the bead bundle, the fitting/removal operations of the tire onto/from the rim may require the use of levers with which it is possible to apply a force sufficient to deform the bead bundle, modifying the configuration from a substantially circular one to an oval one, so as to allow, as mentioned above, the bead to pass over the flange.
To fulfill these functions, the beads are required to have not only sufficient strength to withstand an applied tension but also the rigidity that is necessary to retain the tire on a wheel rim while maintaining a sufficient dimensional precision on the inner periphery of the bead portion to insure good fit to the rim.
In order to satisfy these strength and rigidity requirements, a plurality of high-modulus steel wires have been used as conventional beads. One common type of tire bead is called the single wire tire bead. With a single wire tire bead, a single wire, or coated wire, is wound through a plurality of turns in the angular direction to form a ring shape. FIG. 1 shows a cross section (cross hatching omitted for clarity of illustration) of a single wire tire bead having a hexagonal cross section wire center geometry, with each circle representing one turn of the wire. A ribbon wire center geometry is shown in FIG. 2.
Although steel beads have demonstrated long term commercial success, such conventional beads have had the problem that their weight accounts for about 5% of the tire weight, which is one of the obstacles to the objective of realizing lighter tires that is presently gaining increasing importance.